Plasma processing is widely used in the semiconductor industry for deposition, etching, resist removal, and related processing of semiconductor wafers and other substrates. Inductive plasma sources are often used for plasma processing to produce high density plasma and reactive species for processing wafers. For instance, inductive plasma sources can easily produce high density plasma using standard 13.56 MHz and lower frequency power generators. Inductive plasma sources combined with RF bias have also been used in etchers, for example, to provide independent control of ion energy and ion flux to the wafer.
For certain plasma processes such as a dry strip process for resist removal, it is not desirable to expose the semiconductor wafers directly to the plasma. In these processes, a plasma source is used mainly as an intermediate for modification of a gas composition and to create some chemically active radicals for processing wafers. The plasma is formed remotely from the processing chamber and desired particles are channeled to the semiconductor wafer for example through a grid that is transparent to neutral particles and not transparent to the plasma.
Because rates of processes on the wafer are directly proportional to the rate with which these new species are created and delivered to the surface of the wafer, these processes typically require high RF power (e.g. about 3-5 kW) and in some cases high gas flows (e.g. about 5-25 slm) and high pressure (e.g. about 1000 mTorr). When gas and energy consumption demands are that high, the efficiency of the plasma source becomes increasingly important. Efficiency of the plasma source affects both capital and operational costs. In addition, as a trend of newer and more restrictive government regulations on energy and toxic gas consumption continues, efficiency for both gas and energy consumption will become a more determining characteristic of a plasma source.
Control of the process profile on the wafer is also an important feature of plasma sources. Achieving a good profile, however, often conflicts with achieving increased efficiency. Typical gas injection systems do not provide a high efficiency of gas utilization and create a complicated flow profile that is difficult to control. For example, plasma sources with plasma injection in the center can have a high efficiency of gas utilization, but the profile can be very centered.
Thus, a need exists for a more efficient plasma source that can be used for dry strip processes and other processes where direct plasma interaction with the wafer is undesirable. An efficient plasma source that provides for process profile control across the wafer without change of the hardware would be particularly useful.